1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, facsimile machine, and printer, and to a fixing device employed therein.
2. Description of the Related Art
To date, various devices have been proposed as fixing devices for fixing an unfixed toner image onto a recording medium. One such known device is a belt nip-type fixing device, which forms a nip portion by friction welding an endless belt to a rotatable heating roller with an elastically deformable surface, and fixing a toner image onto a recording medium by passing a recording medium carrying an unfixed toner image through this nip portion.
As an example of this type fixing device, Japanese Patent No. 3298354 proposes a fixing device comprising a pressure pad, which is arranged in a non-rotating state on the inside of an endless belt, provides a nip, which passes a recording medium through a heating roller by elastically deforming the endless belt, and locally elastically deforms the recording medium outlet side of the surface of the heating roller. In this fixing device, by locally elastically deforming the recording medium outlet side of the nip portion of the surface of the heating roller, a recording medium is separated from the heating roller at the nip portion outlet without providing separating means, such as a separating claw or the like. This takes advantage of the fact that the adhesive force of the interface between the fused toner and the heating roller surface is not determined simply by the physical property values of the chemical materials of the two surfaces alone, and is greatly affected by the deformation of the heating roller surface. Specifically, it is a phenomenon in which the adhesive force between the toner and the heating roller surface decreases when transitioning from a state in which the fused toner is making contact with the surface of the heating roller, which has surface deformation from beforehand, to a state in which this surface deformation is instantly cancelled.
In this fixing device, the recording medium outlet side of the nip portion of the heating roller surface is locally elastically deformed, and the toner is fixed onto the recording medium in the vicinity of the nip portion outlet while being subjected to deformation, and the instant this deformation is canceled at the nip outlet, a recording medium is separated from the heating roller by the abrupt drop in the adhesive force of the toner and heating roller surface. Further, this fixing device is advantageous in that, since the pressure pad is used in a non-rotating state and the endless belt is friction welded to the heating roller, heat loss is small compared to when a pressure roller is used in a rotating state.
However, the belt nip-type fixing device of the above-mentioned publication is deficient in that the recording medium outlet side of the nip portion of the heating roller surface is locally elastically deformed, and a velocity difference occurs between the front surface and rear surface of the recording medium at the part subjected to deformation by this local elastic deformation, making image misalignment more apt to occur. Further, since applying pressure to the heating roller using a hard pressure pad member creates a nip shape having a locally small curvature at the surface of the heating roller, the burden placed on the recording medium, specifically, paper damage and amount of curl, is great.
Further, in a fixing device that utilizes a heating roller, using a heating roller with a large surface curvature can enhance separation performance, but the disadvantage is that that device becomes larger in size. Using a heating roller with a small surface curvature has the advantage of compactness, but is apt to give rise to deficiencies that increase the likelihood of image misalignment, paper damage and a large amount of curl.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-open Nos. 02-309376, 2005-077786, 2005-115256 and 2005-208321.